1. Field of the Invention
The present invention relates to super paramagnetic fluids, of the type usually referred to as ferrofluids, having improved thermal and oxidative stability and to a process for making super paramagnetic fluids having improved thermal and oxidative stability.
2. Description of Related Art
Super paramagnetic fluids, which are subsequently referred to as magnetic fluids, are colloidal suspensions of magnetic particles in a carrier liquid. The magnetic particles are suspended in the carrier liquid by a dispersing agent which attaches to the surface of the magnetic particles to physically separate the particles from each other. Dispersing agents are molecules which have a polar "head" or anchor group which attaches to the magnetic particle and a "tail" portion which extends outwardly from the particle surface. The carrier liquid must be a thermodynamically good solvent for the tail portion of the dispersing agent in order to produce a stable ideal colloid of magnetic particles in the carrier liquid.
Magnetic fluids have a wide variety of industrial and scientific applications which are well known to those of ordinary skill in the art. Specific uses of magnetic liquids which illustrate the present invention and its advantages include the use of magnetic liquids as components of exclusion seals for computer disc drives, seals for bearings, for pressure and vacuum sealing devices, for heat transfer and damping fluids in audio speaker devices, and for inertia damping.
Ideally, magnetic fluids suitable for sealing disc drives for computers have a low viscosity and a low evaporation rate. These two physical characteristics of magnetic fluids are primarily determined by the physical and chemical characteristics of the carrier liquid. Magnetic particle size and size distribution and the physical and chemical characteristics of the dispersant, however, also affect viscosity and often the evaporation rate of magnetic fluids.
The characteristics of low evaporation rate and low viscosity are difficult to achieve in a magnetic fluid since carrier liquids having the lowest evaporation rate are usually liquids of high molecular weight. The viscosity of carrier liquids tends to increase as the molecular weight of the liquid increases. In addition, high molecular weight materials, whether polar or non-polar, tend to have lower solubility for the tails of dispersing agents as the molecular weight of the carrier liquid increases.
Magnetic fluids used for inertia damping and similar applications do not require a low viscosity and in fact ordinarily require a relatively high viscosity. Thermal stability of magnetic fluids used in inertia damping equipment is, however, a significant concern.
The selection of a dispersant is a critical factor in providing magnetic fluids which remain stable suspensions in the presence of a magnetic field yet which have desirable viscosity and volatility characteristics. Fatty acids, such as oleic acid, have been used as dispersing agents to stabilize magnetic particle suspensions in some low molecular weight non-polar hydrocarbon liquids such as kerosene. Use of fatty acids, however, has not proven satisfactory for dispersing magnetic particles in polar organic carrier liquids or hydrocarbon oils which are high molecular weight non-polar carrier liquids.
Magnetic fluids using polar organic carrier liquids are disclosed in U.S. Pat. No. 4,430,239 which discloses using phosphoric acid esters as dispersing agents in polar carriers such as di(2-ethylhexyl)azelate. It has been found that the magnetic fluids illustrated in U.S. Pat. No. 4,430,239, however, are thermally and oxidatively unstable at temperatures in excess of about 100.degree. C. In fact, the temperature of the magnetic fluids described in U.S. Pat. No. 4,430,239 are ordinarily maintained below about 80.degree. C. to ensure that the magnetic fluid remains stable. If the temperature of 100.degree. C. is exceeded, the phosphoric acid ester dispersing agent decomposes, resulting in an unstable magnetic fluid in which the magnetic particles begin to agglomerate and precipitate out of the carrier liquid. When the magnetic fluid becomes unstable, the seal is lost since the magnetic fluid is no longer held in place by the magnetic force applied by a magnet. Accordingly, when magnetic fluids such as those illustrated in U.S. Pat. No. 4,430,239 are used in a pressure or vacuum sealing device which is exposed to a source of heat, the apparatus usually includes a cooling system which circulates a cooling liquid, such as water, to remove heat from the magnetic fluid. The need for cooling systems to maintain the magnetic fluid at a sufficiently low temperature to ensure the thermal stability of the magnetic fluid necessarily complicates the construction of the apparatus. Moreover, cooling systems are attended by problems, such as scale formation in passages carrying the coolant liquids, which require maintenance and may result in equipment failure.
The present invention provides thermally and oxidatively stable magnetic fluids. Because of the characteristics of magnetic fluids made in accordance with the present invention, temperatures in devices utilizing these magnetic fluids may exceed 100.degree. C. without impairing significantly the stability of the magnetic fluids. Therefore, the cooling mechanisms used to cool the magnetic fluids in equipment, such as pressure or vacuum sealing devices, may not be required when magnetic fluids of the present invention are used to form the seals.
The present invention also provides a process for making magnetic fluids which are thermally and oxidatively stable and which enables one making magnetic fluids to control other magnetic fluid characteristics such as viscosity and evaporation rate.